Effects of sol properties and calcination on the performance of titania tubular membranes

Journal of Membrane Science 173 (2000) 263–273

Effects of sol properties and calcination on the performance of titania tubular membranes Li-Qun Wu, Pei Huang, Nanping Xu∗ , Jun Shi Membrane Science & Technology Research Center, Nanjing University of Chemical Technology, Nanjing 210009, PR China Received 26 April 1999; received in revised form 20 January 2000; accepted 2 March 2000

Abstract Crack-free tubular TiO2 membranes have been successfully synthesized from colloidal titania sols by the sol-gel technique. Pore sizes of the resulting membrane were mainly controlled by the sol properties and calcination conditions. In order to obtain smaller sol particles, the pH value of titania sol should be kept higher than 2. Methylcellulose (MC) of about 0.1 wt.% in titania sol systems was suitable for use as an organic additive to improve the drying properties of titania gel. Pore sizes of the membrane prepared increased with increasing calcination temperature, especially sharply from 700 to 800◦ C, due to the phase transformation of titania. Pore sizes of the membrane determined by liquid/liquid displacement porometry (LLDP) were in the range of 2–20 nm in this study. Pure water permeability, molecular weight cutoff of polyethylene glycol (PEG) aqueous solution, argon permeability and the separation factor of H2 /N2 mixture through the TiO2 membrane calcined at various temperatures were also measured to investigate the performance of the membrane. The comprehensive experimental data showed that the membranes prepared were crack-free and appropriate for application in liquid and gas separation. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Ceramic membranes; Inorganic membranes; Gas separations; Liquid permeability and separations; Titania

1. Introduction Titania membranes have received significant attention in recent years because of their unique characteristics such as high water flux, semi-conductivity, catalysis and chemical resistance, compared to other membrane materials including ␥-Al2 O3 , SiO2 and ZrO2 [1]. The potential applications of titania membranes are numerous in the ultrafiltration process and in the catalytic/photocatalytic membrane reactor systems for liquid and gas separations [2–4].

∗ Corresponding author. Tel.: +86-25-3319580. E-mail address: [email protected] (N. Xu)

The sol-gel technique is considered to be one of the most practical methods for producing ceramic membranes. The synthesis of crack-free titania membranes by the sol-gel technique was found to be much more difficult than those of other materials such as alumina [1]. For preparing titania membrane by the sol-gel technique, particle size and the organic additives in the sol are the important factors in formatting crack-free membrane with desired pore size. As reported [4–6], the particle size in titania sols was affected by the molar concentration of hydrogen ion, or pH value in the sols. The role of organic additives could also be found in the literature [5,6]; however, the quantitative description of organic additives in a specific case was little reported.

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On the other hand, pore size of the membrane is also controlled by calcination temperature. Many researchers [6–8] observed that the pore size of titania membrane increased with increasing calcination temperature. They attributed the phenomena to the phase transformation of titania, which crystallized with anatase or rutile form. However, the temperature range of the phase transformation reported was not completely consistent in the literature. Kumar et al. [9] studied the phase transformation behavior of supported titania membrane. They observed that, for an 8 h sintering time, titania membranes underwent anatase to rutile phase transformation in the temperature range of 550–600◦ C. Lin and his co-workers [4] found that, for a fixed calcination time (30 h), phase transformation of titania ceramic membrane occurred in the temperature range of 450–700◦ C (anatase to rutile). Peterson et al. [10] believed that the phase transformation of titania occurred when the temperature was up to 300◦ C. Therefore, more research is needed in order to achieve a deep understanding of the effect of temperature on the phase transformation for a titanate system in this study. In addition, methods of nitrogen adsorption (BET) isotherms, molecular weight cutoff, and gas permeability have usually been used to characterize pore sizes of supported titania for nanofiltration and ultrafiltration [1,6,8,10–12]. Since the BET technique was adapted only to the characterization of unsupported membrane, there was no quantitative description on the pore size and pore size distribution (PSD) of the TiO2 membrane in the literature. Liquid/liquid displacement porometry (LLDP) was suggested to well characterize pore sizes of the TiO2 membrane for nanofiltration and ultrafiltration [13]. Furthermore, information on the influence of calcination temperature on pure water flux, gas permeability, and gas separation factor (H2 /N2 ) of the titania membrane was hardly found. The purpose of this work is to investigate the freely controlled pore diameters of crack-free TiO2 membranes on the support of ␣-Al2 O3 by adjusting components of TiO2 sol and the calcination temperature. The crack-free property, pore sizes and the PSD of the resulting TiO2 membranes were determined by gas/liquid displacement porometry (GLDP) and LLDP, respectively. Pure water flux, argon gas permeability and H2 /N2 separation factor of the membrane

calcined at various temperatures were tested to exhibit the liquid and gas permeation performance of TiO2 membranes.

2. Experimental 2.1. Membrane preparation Home-made tubular asymmetric ␣-Al2 O3 microfiltration membranes were used as the support with an inside diameter of 7 mm, an outside diameter of 9 mm and a length of 110 mm. Fig. 1 gives the PSD of the support and shows a mean pore size of about 0.1 ␮m. TiO2 membranes were prepared using a colloidal route of the sol-gel technique. Fig. 2 shows the scheme for the preparation of TiO2 sol by hydrolysis of tetrabutyl titanate (Ti(OBu)4 ). An aqueous-based sol with a TiO2 concentration of 0.05 M was employed in the fabrication of the titania membrane. Nitric acid was used as the electrolyte for the peptization process. Methylcellulose (MC) was introduced into the colloidal solution as an organic additive. All above chemicals (mainly obtained from Tianjin Chemical Laboratory) were of AR grade and used without further purification. Water was deionized and doubly distilled. The particle sizes and their distributions in the TiO2 sol were determined by lighter-scattering particle size analyzer (Coulter LS 230, USA). Titania membranes were fabricated via a dip-coating process on the inner surface of the tubular ␣-alumina

Fig. 1. PSD of ␣-Al2 O3 microporous membranes used as the support of TiO2 membranes measured by GLDP.

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titania membranes were calcined in air at a certain temperature for 2 h with a heating and cooling rate of 1◦ C/min. 2.2. Membrane characterization

Fig. 2. Schematic representation of the preparation of TiO2 sol.

support. Fig. 3 is the schematic representation of the dip-coating process for the TiO2 membrane. In this experimental protocol, alumina tube was dip-coated with TiO2 sol for 1 min and then dried under ambient conditions for 12 h. The coating and drying steps were usually repeated four-fold in order to keep the membrane sufficiently thin and also ensure that the membrane be crack-free. Finally, the supported

The membrane surface morphology was examined by high-resolution scanning electron microscopy (HR-SEM) (JEOL JSM-6300). SEM photographs were used to observe the morphology and to estimate the thickness of the TiO2 membrane. The phase development of calcined membranes at different temperatures was studied by X-ray diffraction (XRD, Rigaku D/MAX-rB diffractometer) with Cu K␣ radiation. Pore size of the tubular TiO2 membrane was determined by means of porometry using a nondestructive low pressure LLDP technique. A schematic of the porometry is given in Fig. 4. The apparatus consists of a pressure transducer, a special stainless steel permeator for tubular membrane, and a capillary tube content gauge. The membrane was first wetted with a liquid that is held in the pores by capillary forces. Another fluid (liquid) acted with increased pressure on one side of the membrane and expelled the former liquid from the membrane pores. The pressure difference (1P) between the two sides of the membrane tube was

Fig. 3. Schematic diagram of the dip-coating process for the TiO2 membrane on the inner surface of the tubular support. (1) Casting-solution tank; (2) soft tube; (3) valve; (4) tubular membrane; (5) glass tube.

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2.3. Pure water permeation measurement

Fig. 4. Schematic diagram of the LLDP apparatus. (1) Gas cylinder; (2) two-stage regulator; (3) trimming valve; (4) three-way valve; (5) atmospheric relief valve; (6) pressure transducer; (7) tank for permeating liquid; (8) stainless steel permeator; (9) atmospheric valve; (10) selector valve; (11) capillary tube content gauge; (12) content gauge; (13) needle valve; (14) container.

needed to expel the former fluid from the pores of the membrane. The fluid began to flow through the biggest pores; with increase in 1P, the liquid was expelled from the largest of pores as they became open for fluid flow. The pore size and PSD of the membrane can be estimated according to the following equation [14]:  f (r) =

Q dQ − d(1P ) 1P



1 r 5 C2

(1)

where f (r) is the distribution function of LLDP, Q the fluid volume expelled from the membrane, 1P the pressure difference, r the LLDP radius, and C2 a constant parameter.

Permeation test was carried out with pure water on a pilot plant working in our laboratory. The apparatus schematic was very simple. A positive displacement pump drove pure water from a feed container to the membrane module. There was a pressure gauge and a back-pressure regulator (BPR) before and after the membrane module, respectively. Experiments were performed at various pressures that were controlled by the BPR. 2.4. Cutoff determination The rejection rates of polyethylene glycol (PEG) molecules with molecular weights from 300 to 20 000 through the resulting membranes were measured. Organic solutes were filtered at an applied pressure ranging from 0.2 to 1.0 MPa at 25◦ C. The feed concentrations of single components were maintained at 500 ppm. Aqueous solutions of PEG were analyzed for concentration using a Total Organic Carbon Analyzer (Model Shimabzu TOC-5000). 2.5. Gas separation efficiency and permeability measurement The experimental apparatus for the gas separation efficiency measurement is depicted in Fig. 5. The membrane was connected to the body of a stain-

Fig. 5. Schematic diagram of the gas-phase apparatus. (1A, 1B, 1C) Gas cylinders; (2A, 2B, 2C) gas regulators; (3) buffer; (4) pressure gauge; (5) permeator; (6) six-way valve; (MFC) mass flow controller; (V) valve; (GC) gas chromatogram.

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less steel permeator. During this protocol, both the upstream and the downstream were maintained at atmospheric pressure. In the upstream, N2 and H2 were assigned equally to the inner side of the tubular membrane and supplied from gas cylinders, and the flow rate of each gas was controlled by mass flow controllers (Models D07-7A/ZM, Beijing Jianzhong Machine Factory, China). In the downstream, the partial pressures of gases are controlled by sweep gas argon with an unchanged flow rate of 20 ml/min, as the sweep gas for the permeating gases, was fed to the support side of the membrane in order to adjust the partial pressure of the downstream. The effluent streams were analyzed by gas chromatography (GC, Model Shimabzu GC-7A), which was equipped with a 2 m 5 A molecular sieve operated at 40◦ C with argon as the carrier gas. The separation efficiency through the membrane was calculated from the data collected. The argon permeability of the resulting membrane was conducted by the experimental apparatus adjusted from Fig. 5. By shutting off V1, V2 and the BPR valve and making argon gas, instead of using the 1C gas cylinder, the apparatus was used to test gas flux of the membrane. The flow rate of the penetrative gas was measured by a mass flow controller (Models D07-7A/ZM, Beijing Jianzhong machine factory, China). Argon permeability data were calculated from the slope of the gas permeation test with the pressure drop range of 0.1–0.3 MPa. During the experiment, the downstream was maintained at atmospheric pressure, and the desired pressure drop was obtained by adjusting the upstream pressure.

3. Results and discussion 3.1. Effect of pH value on the TiO2 particle size Fig. 6 shows the effect of pH value on the particle sizes and their distribution in the TiO2 sol, indicating that an increase in the pH value leads to a smaller particle size. When the pH value was below 2, the particle size of the sol was much larger than that in the case of pH being above 2. So during the experimental operation in this study, the pH value of the solution was initially kept at 2, which was then increased slowly and with sufficient care by trickling the solvent. By doing so, the particle size would get smaller.

267

Fig. 6. Effect of pH value on the particle size of titania colloidal sols. (䉱) pH=1.85; (䊏) pH=2.01; ( ) pH=2.22; (䊉) pH=2.42.

The phenomena mentioned above can be explained by the principle of the hydrolysis reaction of titania sols [10]. When the pH value of the solution was below 2, the solubility of titania increased dramatically with decreasing pH value. The high solubility of titania made it easy to obtain solutions in which all of the titania was present in dissolved form. In contrast, if the pH value was increased to higher than 2, the solubility of titania decreased. After complete hydrolysis of the alkoxide precursor, the excess titania species came forth, which was available for participation in condensation reactions. If the pH value of the sol was increased rapidly, an amount of titania became available for undergoing condensation reactions, which would produce relatively large particles. However, if the pH value of the solution was increased slowly and carefully, the rate of condensation reactions could be controlled precisely, which would lead to small particles. In summary, pH value of the titania sol should be controlled at higher than 2 and must be increased slowly and carefully. Therefore, a relatively smaller particle size of TiO2 sol could be obtained. 3.2. Effect of organic additives on the formation of membranes The drying process was one of the most crucial steps in the formation of ceramic membranes because membranes tend to crack during this process [9]. The additives introduced in a colloidal solution are used to improve drying properties of the gel because it can avoid particle aggregation, adjust the viscosity of

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Table 1 Quality of the TiO2 membrane after calcining with different MC contents in the sol MC (wt.%)

Quality

0

0.1

0.2

0.3

0.4

0.53

Crack

Crack-free

Micro-crack (observed by SEM)

Few fragments

Fragment

Fragment

the sol and increase the strength of the unfired material and prevent crack formation [7]. As reported, PEG [11,15], polyvinyl alcohol (PVA) [4,16] and hydroxypropyl-cellulose (HPC) [6] could be used as an organic additives. For the sol system in this study, MC was found to be very suitable. Table 1 gives the quality of the membrane as MC content in the TiO2 sol was changed. As shown, when the MC content is controlled relatively at about 0.1 wt.%, the composite membranes are free of cracks after drying and calcination. This indicates that a little amount of MC was enough to obtain a perfect membrane, which is consistent with the conclusions reached by Zaspalis et al. [6]. The introduction of measured organic additive to the TiO2 sol can restrain the hydrolysis of the metal alkoxide and raise the rate of the condensation reaction. Therefore, the gel network will be strengthened, which is beneficial for the formation of the membrane. Fig. 7(a) and (b), respectively, show the SEM photographs of the surface and the cross-section of the TiO2 /␣-Al2 O3 composite membrane calcined at 300◦ C. It can be seen that the surface toplayer was homogenous and about 1.5 ␮m thick.

to rutile, which was confirmed by XRD analysis. Fig. 10 shows the XRD patterns of TiO2 samples at different calcination temperatures. As shown, when the calcination temperature was below 500◦ C, the

3.3. Effect of calcination temperature on the pore size In this study, the calcination process was studied in a broad range from 300 to 1000◦ C. Fig. 8 shows the pore sizes and PSD results of the resulting membranes by LLDP measurements at different temperatures. In order to give an obvious observation, Fig. 9 directly shows the mean pore diameters of the TiO2 membrane as a function of calcination temperature, indicating that the pore sizes increased with increasing calcination temperature. As can be seen, the mean pore diameters are less than 5 nm when the temperature is not higher than 700◦ C. Once the temperature is increased up to 800◦ C, the mean pore diameters increase rapidly to about 18 nm. This can be attributed to the phase transformation of the TiO2 membrane from anatase

Fig. 7. SEM photographs of (a) top surface, and (b) cross-section of the supported TiO2 membrane calcined at 300◦ C.

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Fig. 8. PSD results of supported TiO2 membranes calcined at different temperatures measured by LLDP. (䉬) 300◦ C; (䊏) 400◦ C; (夹) 500◦ C; (䉱) 600◦ C; (䊉) 700◦ C; (䉲) 800◦ C; ( ) 900◦ C; ( ) 1000◦ C.

corresponding crystal-type of TiO2 membrane was dominantly anatase-phase, which disappeared between 600 and 800◦ C. Above 800◦ C, the more stable phase, rutile, grew to its fullest. Fig. 11(a) and (b) show the surface SEM image of the composite membrane calcined at 700 and 800◦ C, respectively. Grains with clear grain boundaries of the two titania membranes are visible. Comparing the two photographs, it is obvious that TiO2 grains increase remarkably as the calcination temperature changes from 700 to 800◦ C. So the mean pore diameters of the membranes are certainly increased. In a word, it is the formation of a larger crystal phase (rutile) that led to significant change in the pore structure of the membrane.

Fig. 10. XRD patterns of a TiO2 sample at different calcination temperatures. A: anatase; R: rutile. (1) 200◦ C; (2) 300◦ C; (3) 400◦ C; (4) 450◦ C; (5) 500◦ C; (6) 600◦ C; (7) 700◦ C; (8) 800◦ C.

This was the reason that there existed a leap on the mean diameters of the TiO2 membrane as shown in Fig. 9. This result suggested that the pore size of titania membrane could be controlled by controlling the crystalline phase of titania in calcination. 3.4. Effect of calcination temperature on pure water permeability

Fig. 9. Pore diameters of the supported TiO2 membrane as a function of calcination temperature.

Fig. 12 shows pure water permeation flux as a function of pressure difference, indicating a direct proportional relationship. Permeability of the composite membrane to pure water was calculated by the slope of each curve at different calcination temperatures. Thus, pure water permeability increased with the calcination temperature. There was also a jump in the pure water flux when the temperature was at 800◦ C.

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Fig. 12. Effect of calcination temperature on pure water permeability of the tubular supported TiO2 membrane. (䉲) 300◦ C; (䊏) 400◦ C; (夹) 500◦ C; (䉱) 600◦ C; (䊉) 700◦ C; (䉬) 800◦ C; ( ) 900◦ C; ( ) 1000◦ C.

7 to 70×10−11 m/s Pa. The increased multiple in pure water permeability was not consistent with the Hagen–Poiseuille equation, which indicated that water permeability was proportional to the square of the pore size. This may be attributed to two aspects. One was that the radius in the Hagen–Poiseuille equation was not the exact one from LLDP according to different principles. The other important reason was that, during the process of calcination, other structural parameters of the membrane, which also affect pure water permeability, i.e. porosity, and shape factor, may change simultaneously with change in the pore diameter. 3.5. Effect of calcination temperature on molecular weight cutoff

Fig. 11. Surface SEM photographs of the supported TiO2 membrane calcined at (a) 700◦ C, and (b) 800◦ C.

Compared to Fig. 9, when the sintering temperature changed from 300 to 1000◦ C, the LLDP mean pore size changed from about 2 to 20 nm, while pure water permeability was in the range from about

Fig. 13 shows rejection of the membrane calcined at different temperatures. As shown, molecular weight cutoff values for the membranes calcined at 700 and 800◦ C as defined at 90% rejection were approximately 4000 and 20 000, respectively. The data of the membrane calcined at 700◦ C (pore size: ≈3 nm) coincided with the value of the same membranes with average pore sizes in the range of 3.0–4.0 nm reported by Hyun and Kang [1] (6000–9000) but much lower than that of the silica/zirconia membranes with pore sizes ranging from 1.0 to 2.9 nm reported by Tsura et al. [17] (200–1000). This is because the rejection mechanism of nanofiltration membrane is based on not only a

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flow, Knudsen diffusion, molecular sieving and surface diffusion, by which permselective transport through a porous membrane can take place. Experimental results demonstrated that the mechanism of transport of this experiment dominantly involved Knudsen diffusion. The results coincided with the pore size (<20 nm) of the composite membrane. When the mechanism of transport through the membrane is Knudsen diffusion, the permeability can be of the form [14] Pm = Fig. 13. Molecular weight cutoff curves of the membranes calcined at (䉱) 300◦ C, (夹) 700◦ C, and (䊉) 800◦ C.

‘sieving effect’ but also ‘interaction’ between solutes and the membrane surface [18]. The effect of interaction between –OH and –Si would be stronger than that between –OH and –Ti. 3.6. Effect of calcination temperature on argon permeability Fig. 14 shows the argon permeability of the membrane calcined at different temperatures as a function of the average pressure at room temperature. The data indicted that argon permeability was essentially independent of the average pressure. It is well known that there are four basic mechanisms, including viscous

2εµk vr 3RTL

(2)

where ε is the porosity of the membrane, µk a shape factor for Knudsen diffusion, r the mean radius of the pores in the material, R the gas constant, T the absolute temperature, L the membrane thickness, and v the mean molecular speed given by   8RT 1/2 (3) v= µM where µ is the viscosity of the gas, and M the molecular weight of the permeating molecule. In this study, argon was used and gas permeation was measured at room temperature, so R, T, L, and v in Eq. (2) are fixed. Therefore, the measured gas permeability is only influenced by εµk r. As stated, when the sintering temperature changed from 300 to 1000◦ C, the LLDP mean pore size changed from about 2 to 20 nm. However, argon permeability through the composite membranes increased from 3×10−6 to 6×10−6 mol/m2 s Pa as shown in Fig. 14, which indicated that εµk decreased with increasing calcination temperature. 3.7. Effect of calcination temperature on H2 /N2 separation efficiency

Fig. 14. Effect of calcination temperature on argon gas phase permeability. (䉲) 300◦ C; (䊏) 450◦ C; (䉱) 600◦ C; (䊉) 700◦ C; (䉬) 800◦ C; ( ) 900◦ C; ( ) 1000◦ C.

Fig. 15 shows the separation factor of H2 /N2 as a function of calcination temperature. The separation factor remained basically unchanged with variation in the calcination temperature. As Knudsen diffusion was the only mechanism of transport for this pore size range, it was reasonable that the separation factor of H2 /N2 mixture was kept at a certain level. This indicates that the increasing calcination temperatures hardly influence the H2 /N2 separation factor of the composite membranes prepared in this study. As can

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Fig. 15. Effect of calcination temperature on the H2 /N2 gas separation factor.

be seen in Fig. 15, the value of the separation factor for H2 /N2 is about 3.5, which is slightly lower than the Knudsen ratio (3.74) calculated from Eqs. (2) and (3). The results indicated that some large pores exist in the membrane. Another reasonable explanation for the results is that the Knudsen ratio is theoretically calculated from a single layer of the membrane with a small pore size. In this experiment, however, the TiO2 /␣-Al2 O3 composite membrane consisted of a titania toplayer and an alumina support. The separation factor was also affected by the two parts of the composite membrane. Therefore, the separation factor for H2 /N2 in this work was relatively lower than the Knudsen ratio due to the existence of the support.

4. Conclusions Crack-free tubular titania/␣-alumina composite membranes with a thickness of about 1.5 ␮m were successfully synthesized by the colloidal route of the sol-gel technique. The main conclusions from the research are as follows: 1. Smaller particle size of the sol can be obtained by controlling the pH value at higher than 2. When the MC content was controlled relatively at about 0.1 wt.%, the membranes prepared were crack-free after drying and calcining. 2. The pore sizes increased with increasing calcination temperature due to phase transformation from anatase to rutile occurring in the temperature range

of 500–800◦ C. The mean pore diameters varied by less than 5 nm when the calcination temperature was not higher than 700◦ C. Other structural parameters, i.e. porosity, and shape factor, also changed with change in the pore diameter. 3. Pure water permeability of tubular composite membrane increased as the calcination temperature increased. The permeability ranged from about 7 to 70×10−11 m/s Pa. Molecular weight cutoff data for the membranes calcined at 700 and 800◦ C are approximately 4000 and 20 000, respectively. 4. Argon permeability of the composite membranes ranged from 3×10−6 to 6×10−6 mol/m2 s Pa with increasing calcination temperature. The mechanism of transport through the membrane was dominantly controlled by Knudsen diffusion. 5. H2 /N2 separation efficiency of the composite membranes prepared was almost independent of the calcination temperature. The separation factor for H2 /N2 mixture was slightly lower than the Knudsen ratio due to the structure of composite membranes.

Acknowledgements This work is supported by the Ministry of Science and Technology of China (MSTC, No. 96-A13-01-03).

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